1. Field of the Invention
The present invention relates to semiconductor packages, in particular to those using wire-bonding.
2. Background of the Invention
The technology of wire-bonding has been used for a long time in the manufacture of semiconductor packages.
FIG. 1 represents a cross-section of part of a semiconductor package 1. The semiconductor package has a support or substrate 2 which has on an upper surface attached a semiconductor die 3. On a top surface of the semiconductor die 3, there is arranged a pad 4, known as a bond-pad and of conductive material. Common materials for bond-pads 4 are copper and aluminum. A part of the upper surface of the bond-pad is exposed and this area is delimited by an opening in a passivation layer 5 covering most of the top surface of the semiconductor die.
There is a metal pad 6, commonly called a bond-finger, arranged on the top surface of the substrate.
A metal wire 7, called a bond-wire, connects the bond pad 4 and the bond finger 6. At the end of the bond-wire 7 connected to the bond-pad 4, there is a structure 8 resembling a squashed ball and known as a ball. The method of connecting the bond-wire 7 to the bond-pad 4 involves melting a free end of the wire so that a ball is formed. This ball is then forced down onto the pad while heat is applied. This results in the ball being crushed and a weld being formed at the interface of the ball 8 and the bond pad 4.
The bonding machine (not shown) then forms a loop of the bond-wire 7 down to the bond-finger 6. The bond-wire 7 is then crushed into the bond-finger 6 and a weld is formed at the interface between bond-wire 7 and bond-finger 6. The bonding machine then breaks the bond-wire 7 and the resulting structure is known as a stitch 9.
Hitherto the materiel mostly used for the bond-wires 7 is a high purity alloy of gold. This choice was dictated by following reasons. Firstly, gold is a very good electrical conductor which is useful because this allows bond-wires 7 which are relatively fine diameter (of the order of 20μ or less). Secondly, gold is a soft material which makes it well adapted to the techniques described above. Finally, gold suffers from practically no corrosion, making the wires very chemically stable.
Unfortunately, however, because gold is a very expensive material there is a strong financial incentive to use a cheaper metal. Therefore, recently, attention has been turned to copper and alloys thereof.
It has been possible to accomplish the above described steps using wires based on copper. Copper also fulfils the requirements of electrical conductivity. However copper presents a significant disadvantage in that it does not resist corrosion well. Its oxide is permeable and therefore does not protect the metal from corrosion to the extent that, as long as moisture is available, the corrosion continues.
Furthermore, most semiconductor packages are manufactured using a plastic resin which does protect the semiconductor die and the bond-wire from mechanical damage. However the plastic resin is, itself, also permeable to moisture. The result is that over sufficient time, the corrosion of the copper wires will continue until some of them fail and the device is rendered inoperative. It is found that the lifetime of the device may thus be unacceptably shortened.
This problem can be mitigated by the use of thicker copper wires but this is inconvenient where the devices have a large number of pins.
Attempts have also been made to address this problem by using copper alloy wires which have been previously coated in a protective metal. However, the mechanical action of the bonding process has a tendency to break this protective layer at the points shown by the arrows 7A in FIG. 1. Furthermore, the melting of the end of the wire in order to form the ball has a tendency to consume the coating and leave the ball unprotected.
Since the wires have been subjected to mechanical stress, there are residual stresses remaining in the areas indicated by the arrows mentioned previously. These residual stresses can accelerate the corrosion precisely at those areas where the protective coating has been inadvertently removed.
It is therefore desirable to provide a means that permits the use of bond-wires of inexpensive materials which do not have intrinsically high corrosion resistance yet will be stable over time.